The plasma membrane of the mammalian egg serves important but poorly understood functions during fertilization. For fertilization to occur, the sperm cell must recognize and fuse with the egg plasmalemma. Subsequent to fertilization, the properties of the egg plasma membrane are modified in certain species such that polyspermy, a lethal condition for mammalian embryogenesis, is prevented. Little is known, however, concerning the molecular basis for sperm-egg interaction and polyspermy block at the plasma membrane level. The primary objective is to gain insight into the functional and biochemical properties of the mouse egg plasma membrane. One of the major goals will be the isolation of plasma membranes from zona-free mouse eggs. Radiolabeled lectins will be used as cell surface markers to support plasma membrane isolation by density gradient centrifugation. The second goal will be an evaluation of whether the egg plasma membrane contains sperm receptor activity that is lost or modified to account for the block to polyspermy. Bioassays and radioimmunoassays will be established to measure sperm receptor activity in membrane isolates from unfertilized, fertilized, activated, and enzymatically modified zona-free eggs. The third aim will be to characterize membrane components that are involved in membrane-membrane interaction at fertilization. Enzymatic and chemical treatments will be applied to zona-free eggs to evaluate the role of specific membrane constituents in sperm-egg interaction. Alterations in membrane polypeptides and glycoproteins of unfertilized, fertilized, activated, and enzymatically modified zona-free eggs will be analyzed by one and two dimensional gel electrophoresis, with cell surface radiolabeling and silver staining techniques utilized to detect components that are unique to unfertilized eggs. The successful completion of these experiments will provide new information concerning a critical aspect of fertilization and early embryonic development.